This invention relates generally to a method for automatically charging storage batteries, and in particular, for automatically charging lead-acid storage batteries in a safe and efficient manner.
It is well known that during the process of charging lead-acid storage batteries, particularly during the terminal stages of the charging process, chemical reactions occur within the cells of the storage battery which produce gases which generally escape to the surrounding atmosphere, creating an irritating and unsafe environment in which to work. In order to reduce such "gassing",the charging current level used is generally lowered, particularly during the final stages of the charging process, to a value substantially below that which would permit recharging to take place in a minimum amount of time. In most cases, particularly when the initial battery voltage is low, the maximum charging current permissible during the final or finishing stages of charging is generally maintained at a rate as low as one tenth of that which would be permissible during the initial stage.
Failure to follow this practice can create both excessive quantites of gas emissions, as well as excessive temperatures within the battery. These problems, if unchecked, can lead to excessive water loss, plate buckling and other deleterious effects within the battery.
One apparatus which has been used to reduce these problems, while also maintaining a sufficient charging rate, is known as a taper charger. A taper charger generally comprises a transformer, a rectifier and an impedance, which impedance causes the charging current of the charger to fall as the output voltage increases, thus avoiding the above mentioned effects. Although taper chargers are relatively safe devices, such a tapered output provides, for most batteries, a less than optimum variation during the charging process.
Another process, known as the two rate method, has also been used. In practicing this method, the charging current is maintained at a constant, maximum value throughout the early portion of the charging process, and reduced comparatively steeply toward the end of the charging cycle when gassing is most prevalent.
Various other charging methods have been proposed for stablizing or controlling charge current rates. One proposed charging method comprises three stages. During the first stage of the process, a relatively high charging rate is used, during which stage the current is held constant against variations in line or battery voltages, or other factors. During the second stage of the process, which starts when the battery is gassing at a moderate rate and has reached a predetermined voltage, typically 2.35 volts per cell in the case of a lead-acid battery, the battery voltage is held constant and the charging current is diminished. During the final stage of the charging process, when the charging current has dropped to a predetermined finishing rate, the current is held constant at the finishing rate until the charge is terminated by a timing device, which continues for a duration based upon either a total charge time, or a predetermined charge time at the finishing rate.
Other attempts have recently been made to overcome the above mentioned problems. For example, in U.S. Pat. No. 4,146,839, issued Mar. 27, 1979, the achievement of a preselected voltage value, typically 2.35 volts per cell, is used to trigger an internal staircase function which automatically drops the voltage, as well as the input charging current, periodically until the finishing current is reached and the battery achieves its final output voltage. However, this procedure has not been found to be entirely satisfactory when the total charging time, or total gas emission, are considered to be important factors.
However, these factors are often critical to the proper operation of certain pieces of equipment. It is also often desirable to reduce the charging time as much as possible, often to a time which is substantially below the standard 8 hour time period typically required for charging larger batteries. It is also generally desirable to limit the quantity of gases emitted during the charging cycle since this not only serves to reduce a substantial explosion hazard, but also reduces the frequency with which water which must be added to the cells of the battery to maintain the electrolyte volume at the level required for satisfactory performance. Lastly, it is at all times desirable to keep the internal battery temperature as low as possible to avoid thermally stressing the components comprising the battery.
One method which has been proposed to obtain a safe and effective charging rate which can fully charge a battery in a relatively short period of time is described in a report published in the Soviet Journal of Technical Economic Information, Vol. 1, p. 5 (1977), by Maslov and Lisovskij. In performing this method, an initially high, constant current rate is applied to the battery. When the final, finishing stage is reached, a finishing charge is applied to the battery. Also during the finishing stage, the battery is periodically discharged. Described is a discharge rate having a duration of a few seconds and occurring at intervals of approximately 5 minutes. Although such a method appears to provide improved results, as well as substantially reducing charging time, the apparatus required and described in the article for accomplishing this method is both bulky and expensive.
It therefore becomes desirable to develop a method which satisfies the aforementioned problems and which permits a relatively short charging time, but which does not require the use of bulky, complex or expensive equipment.